The present invention relates to a method for calendering a paper web or an equivalent web material in a calender, wherein the web material to be calendered is passed through nips formed by a variable-crown upper roll, a variable-crown lower roll, and by intermediate rolls arranged between the upper and lower rolls. The rolls are arranged as a substantially vertical stack of rolls.
The invention also relates to a calender that makes use of the method, and includes a variable-crown upper roll, a variable-crown lower roll, and a number of intermediate rolls arranged between the upper and lower rolls. The upper roll, lower roll and intermediate rolls are arranged on the frame of the calender as a substantially vertical stack of rolls whereby the rolls are placed one above the other, and such that calendering nips are formed between adjacent rolls.
The set of rolls in a conventional supercalender device comprises a number of rolls arranged one above the other to form a stack, i.e., as a stack of rolls. Adjacent rolls, placed one above the other, are in nip contact with each another, and the paper or board web, or equivalent web material to be calendered, is arranged to run through the nips between the rolls. The rolls in the set of rolls are journalled revolvingly on bearing housings which are typically attached to base parts. The base parts are arranged slidably on vertical guides provided in the frame of the calender. Further, the base parts are provided with backup parts arranged on vertical lifting spindles situated in the frame of the calender. One particular function of the lifting spindles is to act as guides in order to keep the rolls in the set of rolls in the correct position. The bearing housings of the rolls in the set of rolls are not fixed rigidly to the frame of the calender. Thus, the bearing housings and the rolls can move in a vertical direction.
Since the combined mass of the bearing housings of the rolls and the auxiliary equipment attached to the bearing housings are quite large, in conventional supercalenders this weight produces a significant drawback in that the combined mass of the bearing housings and the auxiliary equipment attached to the bearing housings produce distortions in the distributions of the linear loads in the nips. Thus, the linear load is not uniform in the nips, but rather there is a considerable deviation in the profile of the desired and applied linear loads at the ends of the nips.
Since a number of rolls are placed one above the other in the sets of rolls in conventional supercalenders, as discussed above, this has the further consequence that the deviated linear loads in the individual nips are cumulative and produce a considerably large error in the overall linear load. This defective distribution of linear load deteriorates the quality of the calendered paper or equivalent web material.
One prior art device intended to resolve the problem stated above is described in the assignee's Finnish Patent No. FI 81,633 (corresponding to U.S. Pat. No. 4,901,637, the specification of which is hereby incorporated by reference herein), wherein the set of rolls is provided with relief means supported on the base parts of the rolls, on one hand, and on spindle nuts provided on the lifting spindle, on the other hand. In this manner, the relief means substantially eliminate distortions arising from the weight of the bearing housings of the rolls and the auxiliary equipment attached to same, e.g., the take-out leading rolls, in the lateral areas of the profiles of linear loads between the rolls. Also, in conventional machine calenders, a device is known in the prior art in which the rolls of the machine calender are provided with a relief system, in particular with hydraulic relief cylinders, in order to eliminate the point loads arising from the bearing housings of rolls and from their auxiliary equipment.
In machine calenders, it is easy to provide such relief means, because the rolls in the set of rolls in a machine calender are arranged by means of linkages mounted on the frame of the calender. However, the use of devices corresponding to those of machine calenders in supercalenders is quite difficult because of the constantly varying diameters of the fiber rolls and because of the high number of rolls.
Owing to the conventional construction described above, another significant drawback of conventional supercalenders relates to the vertical movements of the rolls in the set of rolls. As described above, the bearing housings of the rolls in the set of rolls are mounted on base parts which move vertically along the guides provided in the frame of the calender. This second drawback stems from the friction at the guides which is effective between the guides and the base parts.
As a result of the friction at the guides, the rolls in the set of rolls cannot move freely to be positioned vertically in a desired position. This inability to be completely and freely movable may produce disturbances in the operation of the calender, together with considerable local errors in the distributions of the linear loads. In order to eliminate the friction forces at the guides, in supercalenders, it might be possible to consider the use of the arrangement described above and commonly known from machine calenders, in which the rolls are placed on the frame of the calender by means of linkages mounted on the frame. However, the use of such an arrangement in supercalenders is limited by the fact that the set of rolls in a supercalender includes a number of fiber rolls, whose diameter may vary considerably. As a result of the variation in the diameters of the rolls, in such a case, the rolls must be able to move considerably in the vertical direction. Thus, if the rolls were attached to the frame of the calender by the linkages, the vertical shifting of the rolls would also result in a considerable shift in the transverse direction.
In view of solving the problem described above, in the assignee's Finnish Patent No. 83,346 (corresponding to U.S. Pat. No. 5,038,678, the specification of which is hereby incorporated by reference herein), an arrangement is described to eliminate the friction forces at guides and relieve the axle journal loads arising from the bearing housings of the rolls and from the auxiliary equipment in the set of rolls so as to straighten the distribution of the applied linear load. In FI 83,346, this is accomplished so that the base parts of the intermediate rolls in the stack of rolls in the calender are supported on the lifting spindles so as to be vertically displaceable by means of pressure-medium operated relief devices. The relief devices are arranged between the base parts and the spindle nuts in order to relieve the axle journal loads of the rolls. The bearing housings of the intermediate rolls are attached to the base parts pivotally in relation to an articulation shaft parallel to the axial direction of the rolls. The bearing housings are supported on the base parts and/or on the frame of the calender by means of attenuation devices so as to equalize the forces arising from the movements of the nips between the rolls and to attenuate the vibrations of the rolls.
The devices in the prior art described above involve the drawback that, in the supercalender, the nips are loaded by the gravity of the set of rolls itself, i.e., gravitational forces acting on the weight of the roll. In this case, the distribution of the linear loads from the upper nip to the lowest nip is substantially linear and increasing. This has the consequence that the linear load present in the lowest nip determines the loading capacity of the calender. Thus, the calender is dimensioned in accordance with the loading capacity of the lowest rolls. However, it is a significant drawback that at the same time, some of the loading or calendering potential of the upper nips remains unused.
FIG. 1A illustrates this lost loading or calendering potential of the upper nips. The stack of rolls in the calender is denoted with reference numeral 1. The rectangle drawn alongside the stack of rolls is denoted with reference I and illustrates the calendering potential of the calender, while the horizontal axis of the rectangle represents the linear loads in the nips in the stack of rolls 1. The shaded area in the rectangle, which is denoted with reference A.sub.1, represents the range of linear loads employed in conventional embodiments. As shown in FIG. 1A, the distribution of the linear loads from the upper nip to the lowest nip is a substantially linear distribution which increases toward the lowest nip.
The range of adjustability of the linear loads is quite narrow. The designations B.sub.1 and C.sub.1 indicate those areas in the range of linear loads that remain fully unused in the prior art devices. Since the masses of the rolls in the set of rolls load the nips, regulation of the linear loads to the range B.sub.1 is nearly impossible because high linear loads are unavoidably produced in the lower nips. Thus, it is quite difficult to conduct a running of matt grades with a conventional supercalender if the same machine is used for the production of glazed grades. On the other hand, the range C.sub.1 remains unused because the calender is dimensioned in accordance with the loading capacity of the lowest rolls. Thus, as shown in FIG. 1A, a substantial proportion of the loading capacity of the upper nips remains unused.
In the past, attempts have been made to solve this considerable drawback of unused loading capacity present in the conventional prior art devices. In particular, attempts have been made to increase the deficient loading of the upper nips by placing the supercalender in the horizontal plane or by dividing the stack of rolls in the calender into two roll stacks. In the situation of a horizontal positioning of the rolls of the supercalender, slim chilled rolls and fiber rolls are used, however, it is a drawback in this embodiment that the rolls "hang" down out of the plane of the calender. Further, since the forms of the deflection lines of chilled rolls and fiber rolls are different, this "hanging" is different in comparison between adjacent rolls.
It should be stated further that rapid opening of a horizontally arranged supercalender is highly problematic. A stack of rolls divided into two parts solves the problem of incomplete loading just partially, but not entirely. Such an embodiment is also very expensive, because a calender in two parts requires a higher number of variable-crown rolls (at least 3). There are also several systems of different types based on the relief of the axle journal loads, by whose means the border line between the areas A.sub.1 and C.sub.1 of the calendering potential I illustrated in FIG. 1A can be made steeper. However, none of the existing systems eliminate the increase in the linear load towards the lower nip produced by the masses of the rolls in the supercalender.